Dissertations/Theses - Department of Mechanical Engineering

Molecular dynamics study of heat transfer characteristics of bubble induced nanochannel flow

Mon, 07 Apr 2025 00:00:00 GMT

Molecular dynamics study of heat transfer characteristics of bubble induced nanochannel flow Nasim Hasan, Dr. Mohammad; Nurannabi Miah, Md; 0423102078; 621.011/NUR/2025 The study of phase change heat transfer in liquids has become increasingly significant due to the growing demand for efficient thermal management in compact systems, particularly in applications requiring high heat flux in small-scale devices. However, addressing the challenges posed by small dimensions and high heat densities requires advanced approaches. Boiling heat transfer in bubble induced nanochannel flows has thus emerged as a critical area of research. This study utilizes molecular dynamics simulations to examine the effects of surface interaction potential (W), channel width (D), heater intensity (T) and length (H) on phase change heat transfer in nanochannels. The findings reveal that higher heat source temperatures under hydrophilic conditions promote earlier bubble nucleation, while bubble propagation transitions from gradual at lower temperatures to erratic at higher ones. Multiple nucleation sites are activated for bubble nucleation for a higher heater length due to the lack of accumulation of heat within a single region, as occurs for the case of lower heater length. During heating, a reverse liquid flow towards the liquid pool is observed, driven by a sudden temperature surge at the heater. Additionally, higher surface interaction potentials lead to increased liquid atom accumulation at the heater surface, enhancing energy transfer and bubble propagation, thereby improving heat transfer efficiency. In contrast, under hydrophobic conditions, bubble nucleation is delayed, and the vapor shielding effect which is the formation of a vapor layer between the heat source and liquid, becomes more prominent. This insulating barrier significantly reduces heat transfer efficiency. For identical conditions, hydrophilic surfaces exhibit up to 90% enhancement in heat transfer compared to hydrophobic surfaces. Location of bubble nucleation shifts from middle of the channel to the closest of the heater wall as the wetting condition changes from hydrophilic to hydrophobic. After forming stable bubble nucleus, it propagates gradually in both directions, with upward propagation dominating over time, due to the cooling effect of liquid pool. Lower channel width facilitates bubble nucleation earlier due to the distribution of available heat within a small number of liquid atoms compared to the higher width. By reducing the channel width, nucleation time can be significantly advanced without changing the heating condition and wetting condition. These findings provide valuable insights into phase change heat transfer in bubble induced nanochannel flows, underscoring the pivotal roles of heat source temperature and surface wetting conditions regarding thermal performance of the system.

Numerical investigation on effects of microporous transport layer in polymer electrolyte membrane water electrolyzers

Tue, 07 Jan 2025 00:00:00 GMT

Numerical investigation on effects of microporous transport layer in polymer electrolyte membrane water electrolyzers Aman Uddin, Dr. Md.; Imtiaz Rais, Ahmed; 0421102006; 621.10212/IMT/2025 The Polymer Electrolyte Membrane Water Electrolyzer (PEMWE) is pivotal for efficient green hydrogen production through electrolysis. Cost is a major barrier to the widespread commercialization of PEMWE. Introducing a microporous layer (MPL) into PEMWE has immense potential to improve its performance, which could eventually reduce the overall cost. This study numerically investigates the effects of MPL on PEMWE performance after thorough validation of the model with recent experimental data. The study considers the role of MPL in liquid, gas, and charge transport, as well as the energy interactions within the system under various operational conditions. The findings indicate that the implementation of MPL in PEMWE improves liquid, gas, and charge transport, especially at higher current densities. Moreover, the study reports that the MPL enhances capillary pressure distribution, liquid water saturation, and the dissolved water content in the porous media. The MPL is found to significantly accelerate electrochemical reaction kinetics by increasing the triple-phase contact area. The findings also highlight the enhancement of gas pressure distribution across PEMWE due to the MPL. Additionally, lower MPL thickness, higher permeability, and increased exchange current densities are favorable for amplifying PEMWE performance. In essence, this study reveals the core mechanisms and interactions of governing parameters that optimize the performance of PEMWE with MPL.

CHARACTERIZING THE ROLE OF ELECTRON-PHONON INTERACTIONS IN ULTRAFAST LASER ABLATION OF METAL

Tue, 04 Mar 2025 00:00:00 GMT

CHARACTERIZING THE ROLE OF ELECTRON-PHONON INTERACTIONS IN ULTRAFAST LASER ABLATION OF METAL Arafat Rahman, Dr. Kazi; Mustakim Hayder, Md.; 0423102057; 621.366/MUS/2025 This thesis investigates electron-phonon interactions in ultrafast laser ablation of nickel using hybrid two-temperature model with molecular dynamics (TTM-MD) simulations. The functional definition of subsystem properties and electronic-lattice coupling influences the accuracy of these simulations. The first phase determines the optimal description of thermophysical properties in the electron subsystem by comparing empirical definitions and Density Functional Theory (DFT) based values within the Two-Temperature Model (TTM). Results showed that with simplistic Beer-Lambert’s optical absorption modeling, empirically derived temperature-dependent parameterizations matched previous studies. Subsequently, hybrid TTM-MD ablation simulations were performed using two electron-phonon coupling approaches: temperature difference scaled coupling and Langevin thermostat. Analyses indicated that temperature difference scaled coupling leads to increased tensile stress due to artificial enhancement of the collision cascade, based on the deterministic nature of the coupling force. In contrast, the Langevin thermostat’s probabilistic nature predicts ablation threshold (115 mJ/cm², absorbed fluence) and phase explosion onset (270 ~ 300 mJ/cm²) for 1 picosecond laser pulses more accurately. Building on these insights, the second phase employs a sophisticated TTM-MD framework incorporating Generalized Langevin Dynamics (GLD) for wavevector-dependent coupling and a Helmholtz solver for precise optical absorption modeling. This optical model addresses the inferior performance of DFT-derived electronic properties, revealing that Beer-Lambert's law concentrates energy deposition near the surface, causing inaccurate elevated temperatures. The primary contribution of this research is the first implementation of TTM-MD with GLD coupling for laser ablation modeling, revealing anisotropic effects in ablation characteristics through phonon mode-dependent interactions. This approach predicts an ablation threshold of 300 mJ/cm² (incident fluence), closely aligning with experimental results. The model shows a yield decrease of approximately 20% when the laser propagates along the <110> versus <100> crystallographic direction at near-threshold fluences. These findings enhance understanding of electron-phonon interactions in ultrafast laser processes and highlight the crucial role of crystallographic orientation in ablation outcomes, with potential implications for optimizing laser ablation processes.

Thermophysical and phase change characteristics of R152a/R1234ze(E) refrigerant blends through molecular dynamics simulation

Tue, 11 Mar 2025 00:00:00 GMT

Thermophysical and phase change characteristics of R152a/R1234ze(E) refrigerant blends through molecular dynamics simulation Nasim Hasan, Dr. Mohammad; Aminul Islam, Md.; 0422102048; 621.56/AMI/2025 The study of refrigerants and their blends has always been important due to their wide use and environmental impact. In this study, the thermophysical properties and nanoscale phase change characteristics of R152a/R1234ze(E) blends with varying molar fractions have been investigated using non-equilibrium molecular dynamics simulation (NEMD) under non-equilibrium heating conditions. In total, five different compositions of the refrigerant blend have been considered with the blends containing 100%, 67%, 50%, 33% and 0% molar fraction of R152a. Thermophysical properties such as density, liquid thermal conductivity and isobaric specific heat have been computed for the refrigerant blends considered. To study phase characteristics a three-phase domain is considered where liquid and vapor refrigerant molecules are placed over the solid platinum (Pt) surface. Two different heating conditions for the wall have been applied to induce various phase change modes. The first condition involved a linear increase in wall temperature, where temperature rise rates of 80, 160, and 240 K/ns are employed. The second condition maintained constant wall temperatures of 300 K and 320 K respectively. The phase change characteristics of different refrigerant blends under various heating conditions have been reported utilizing various parameters such as net evaporation number, time averaged heat flux, bubble growth, mean square displacement, and energy contours. Three distinct boiling modes are observed, depending on the heating rate and the mixture composition of the refrigerant blend. Moreover, under isothermal heating conditions, diffusive evaporation of the refrigerant blends took place. The results indicate that the increase in percentage of R152a in the refrigerant blend enhances the heat transfer performance. However, considering both environmental impact and thermal performance, refrigerant blend with %R152a of 67% exhibit better overall performance. Furthermore, if environmental considerations are stricter refrigerant blend up to 50% R152a percentage can be considered without significant decline in phase change performance. In addition, in this study the bubble nucleation process and interfacial characteristics of the refrigerant blends have also been examined to elucidate the mechanisms behind the phase change process. The interfacial behavior is analyzed in terms of interfacial thermal resistance, potential energy contours, and spatial density distribution. It is found that R1234ze(E) molecules exhibit a stronger attraction to the solid surface, resulting in lower interfacial thermal resistance.